The present invention relates to a delay line for a travelling wave tube.
It is known that, in these tubes, the delay line ensures interaction between an electron beam which is focussed along the axis of the line and an electromagnetic wave which is propagated along the line. When the conditions of synchronism of the wave and of the beam are achieved, the electrons give up energy to the electromagnetic wave.
Two types of delay line are known which are:
entirely metal delay lines such for example as coupled cavity lines or ceiling and ring lines;
delay lines comprising a central conductor mounted in a cylindrical sleeve by means of dielectric rods, such for example as helical lines of the "ring and bar" type and the "ring and loop" type.
The problem which arises is that none of these types of delay lines gives entire satisfaction.
The delay lines comprising dielectric rods are more widely used than the entirely metal delay lines for:
their cost of production is lower;
their ultra high frequency characteristics are better, in particular, the pass-band and the coupling impedance are higher;
their small external diameter allows the beam to be focussed along the axis of the line by using a succession of permanent magnets with alternate orientations. This method of focussing the beam--by means of a periodic--permanent magnet structure or P.P.M.--is interesting because of the space saved, its lightness and its zero energy consumption.
The disadvantage of delay lines comprising dielectric rods is that:
these lines are not suitable when the peak power and the average power become great for their central conductor cannot sufficiently remove the heat produced by the electrons which it intercepts and the heat produced by Joule effect losses due to the electromagnetic wave;
moreover, the Applicant has noted that when the electrons of the beam are highly grouped together in packets, that is to say towards the end of the travelling wave tube, focussing of the beam, which is achieved by means of a succession of permanent magnets with alternate orientations, is disturbed, and all the more so since the electronic efficiency of the tube is high.
Entirely metal delay lines may be used without problems when the peak power and the mean power become high but they present drawbacks in so far as the following are concerned:
their cost of production;
their ultra high frequency characteristics;
their focussing which can only be achieved by means of a solenoid, for their external diameter is too great for the field created along the axis by a succession of permanent magnets, with alternate orientations, to be sufficient and the distance along which the magnetic field is to be created is too great for a single permanent magnet to be able to be used.
The present invention relates to a new delay line structure which resolves the problem arising with the two known types of delay line.